Today, adhesion of the hybrid cords comprised of aramide and aramide-Nylon 6.6 fibers is performed by using primary surface activating dipping solution before the main (secondary) dipping in addition to the dipping solution used for adhesion of Nylon 66 cords. The chemical increasing the wetting ability of the surface and the chemical increasing the surface activity are dissolved in water during primary dipping within the surface activating dipping. In secondary dipping, the dipping solution (RFL) comprised of resorcinol, formaldehyde and latex mixture is used. Furthermore, there are applications wherein the carbon black dispersion is added into the secondary dipping.
In the state of the art, after the primary dipping, heat treatment is applied for 60-75 seconds at 240-249° C. After the secondary dipping, the cords are treated with heat treatment for 60-75 seconds again at 240-249° C.
All these processes are performed to provide the adhesion of the fibers used as rubber reinforcing material onto the rubber. For this reason, in order to provide the desired adhesion, the fibers are coated with special dipping, chemicals and cooked by dipping. Aramid and aramid-Nylon 66 hybrid cords dipped with the said method exhibit 60-70% lower adhesion than the cords comprised of Nylon 66 fibers. This ratio is not enough especially in aramid and aramid-Nylon 66 hybrid cords used as cap ply strip in pneumatic tires. Because of the inadequate adhesion, layer separation at high speeds tire rupture accordingly is seen in tire cap ply.
It is not possible to manufacture bias truck tire with aramid and aramid-Nylon 66 hybrid cords dipped with the methods known today. Low adhesion values of the said cords limit the field of use as rubber reinforcing material.
The adhesion of aramid and aramid-Nylon 66 hybrid cords onto the rubber is partially provided with the known technique. By this means, in various applications, the said cord structures are used as reinforcing materials. The surface of the cords are activated with the surface activating material (epoxy) in primary dipping, the fibers are enabled to form chemical bond with the RFL solution in secondary dipping. Carbon black dispersion also increases the adhesion mechanically.
In the state of the art, the material increasing the wetting, ability of the surface (aerosol) used in first dipping decreases the activity of the surface activating material (epoxy) present in first dipping solution, and causes the RFL to break and separate from the cord surface after heat treatment process. In addition to surface wetting agent, caustic (NaOH) solution used for adjusting the pH of the first dipping solution also decreases the activity of the epoxy and reduces the surface adhesion ability. Furthermore, crustation and cracking is seen on the surface of the cord treated with heat for long time at high temperature after the primary dipping, and the crusted layer breaks and separates from the surface of the cord.
Besides, in the state of the art, the particle size of the carbon black dispersion in secondary dipping is in the range of 3-4 mm. Since this particle size cannot provide homogenous. distribution, the activity of the carbon black particles expected to increase the adhesion mechanically decreases. This causes the adhesion values on the cord treated with dipping to vary along the cord. High temperature and long term heat treatment applied after the secondary dipping cause the. RFL layer comprising carbon black to be cracked and crusted and separated from the cord surface. In RFL solutions not comprising carbon black, the said negative effect is seen 40-50% more.